This invention relates generally to three-phase motors and more particularly to apparatus for preventing damage to such motors and loads driven by such motors upon miswiring of the motor or otherwise losing a phase during normal operation.
In a three-phase system, each phase is separated from the following phase by 120xc2x0. If the first phase is designated xe2x80x9cAxe2x80x9d, the second phase, or xe2x80x9cBxe2x80x9d will peak 120xc2x0 after xe2x80x9cAxe2x80x9d has done so. In turn, the third phase xe2x80x9cCxe2x80x9d will peak 120xc2x0 after xe2x80x9cBxe2x80x9d, and 240xc2x0 after xe2x80x9cAxe2x80x9d. 120xc2x0 after xe2x80x9cCxe2x80x9d has peaked, xe2x80x9cAxe2x80x9d will peak once again, completing the xe2x80x9ccirclexe2x80x9d at 360xc2x0 from xe2x80x9cAxe2x80x9d to xe2x80x9cAxe2x80x9d.
A three-phase electric motor requires this 120xc2x0 phase-sequence to operate efficiently. The loss of a single phase would cause motor current and, in turn, temperature to rise dramatically. Such a loss could also cause excessive mechanical vibrations within the motor itself. These vibrations could be transmitted to whatever is being driven by the motor, such as a compressor. In any case, a single-phase loss could result in severe motor damage and possible compressor damage, as well.
Such a situation can be avoided by providing a protection scheme in which a single-phase loss would result in all power being cut to the motor. The use of a dedicated electronic motor protection device is a common means to this end. A control unit within this type of protector monitors the three phases in an electric motor, and determines if a fault condition exists. There are several methods by which the three phases can be coupled to the control unit. However, utilizing current toroids as sensors has gained favor within the industry. Current toroids isolate the control unit from a direct electrical connection with the high voltage of an electrical motor. The output signals generated by this type of sensor can easily be interpreted, as well. This allows the protector to respond to other failure modes beyond phase loss. Such additional features are important in an effective motor protector.
A primary disadvantage of using current toroids in a design is cost. The assembly process needed to ensure consistency of electrical characteristics is complex. The core material in its raw form is relatively expensive, as well. A typical three-phase protection module uses a toroid for each phase. Three of these toroids can account for a large percentage of a completed module""s production cost.
It is an object of the present invention to provide an effective, reliable three-phase motor protection module but one that is more economical than the prior art approach described above.
As noted above, each of the three waveforms in a three-phase motor is separated by 120xc2x0. A loss of one phase will change the relationship between the remaining two phases, each becoming an inverse of the other. In other words, they will be separated by 180xc2x0. This has been demonstrated by extensive testing utilizing a three-phase electric motor wired in a wye configuration coupled to a scroll compressor. Oscilloscope measurements show that the normal 120xc2x0 relationship between the two remaining phases shifts to 180xc2x0 within one cycle after a single-phase loss. Briefly, in accordance with the invention, two toroids are used to monitor a three-phase motor instead of the usual three. Motor current through phases A and B is monitored by a toroid assigned to each of those phases. Phase A or B current loss is directly detected in a conventional manner. However, Phase C has no toroid to monitor its current. In accordance with the invention, phase C loss is detected by indirect means, i.e., the phase relationship between xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d.
Current is induced into the phase A and B toroids from the motor supply lines. Load resistors at the output of each toroid reduce the resulting sine wave to a manageable level. Each waveform is sent to an input of a high-gain, inverting amplifier section. The resulting inverted square wave outputs are much easier for the following logic circuits to interpret than the original sine wave. Both xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d square wave signals are then sent to separate channels of a micro-controller for evaluation with the phase relationship intact. The circuitry within the micro-controller compares the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d signals by means of a firmware-encoded logic AND circuit. This AND-ed output is evaluated by the encoded firmware, as well.
In a normal-run situation, AND-ing both phases produces a square wave with a 16.66 percent duty cycle. This is because the 120xc2x0 phase difference allows both inputs to overlap 16.66 percent of the time. Assuming no other faults are detected, the micro-controller allows the motor under its control to remain operational.
However, a phase C loss will change the 120xc2x0 phase A-to-B relationship. In this situation, one phase becomes a xe2x80x9cperfectxe2x80x9d inverse to the other, a 180xc2x0 difference. Therefore, the waveforms never overlap. The AND-ed result is a digital xe2x80x9clowxe2x80x9d, 100 percent of the time. The micro-controller interprets this constant low as a phase C loss, opening the module relay, and therefore the motor contactor, after a predetermined time.
Since there is no such thing as perfect, from a practical standpoint, one must expect that a phase C loss will not result in an absolute 180xc2x0 phase shift between xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d. Even a small deviation from this relationship would cause xe2x80x9cspikesxe2x80x9d in the output of the AND circuit. This could result in delayed phase C loss detection, or no detection at all. The design firmware has been encoded to counteract this situation. An AND output that results in less than a two-percent duty cycle is treated as a phase loss. This ensures that small differences in electrical characteristics between the toroids, or among other circuit elements, will not prevent phase C loss detection.
The phase loss detection offered by the motor protector made in accordance with the invention will protect a motor wired in either a wye or delta configuration, where a single-phase loss would reduce the current paths from three to one. However, phase C loss in a wye system with a Neutral return, an uncommon configuration, would be undetectable by the motor protector.